Electrochemical cell

ABSTRACT

An electrochemical cell that converts chemical energy to electrical energy includes a cathode with an active material of fluorinated carbon on a perforated metal cathode current collector, a lithium anode on a perforated metal anode current collector, a stepped header, a stable electrolyte, and a separator. In various embodiments, an anode current collector design, a cathode current collector design, a stepped header design, a cathode formulation, an electrolyte formulation, a separator, and a battery incorporating the electrochemical cell are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.15/924,158, filed on Mar. 16, 2018, which claims priority to U.S.provisional application No. 62/472,522, filed on Mar. 16, 2017 under 35U.S.C. 119(e), the contents of which are hereby incorporated byreference in their entirety.

BACKGROUND

The present inventions relate generally to the field of electrochemicalcells. More particularly, the present inventions relate tolithium/fluorinated carbon (Li/CF_(x)) electrochemical cells for use inimplantable medical devices.

Li/CF_(x) electrochemical cells are known to be used in multitude ofdevices including implantable medical devices. These electrochemicalcells are known to swell during discharge. However, in the design of amedical device, more particularly an implantable medical device, theswelling may need to be controlled. The control on swelling may beneeded to ensure that enough space is reserved for the cell volumechange in order to prevent damage to the device circuitry. Consequently,the more swelling the cell experiences, the more void space may beneeded to be reserved in the device, leading to greater total devicevolume. The swelling may result in capacity loss due to lack ofelectrolyte in contact with some solid particles (that is, loss ofinterface between solids and electrolyte).

In the art there are references to the minimization or elimination ofswelling in Li/CF_(x) cells discharged under high rate applications.When CF_(x) materials are synthesized from fibrous carbonaceousmaterials, in comparison to petroleum coke, cell swelling may be greatlyreduced, and in some cases eliminated. It is believed that the Li/CF_(x)cell is known to produce a cathode swelling that may result inmechanical deformation of the cell. References in the art report thecathode swelling as a function of discharge depth, rate, andtemperature. A mechanism in which the discharge product is LiF depositedon the internal surfaces of the carbon layers left behind afterelectrochemical reduction with this deposition leading directly to themeasured cathode swelling is also proposed in the art.

In view of the foregoing, it is clear that these traditional techniquesare not perfect and leave room for more optimal approaches.Particularly, in the field of implantable medical devices, a smallertotal device volume may be desired and hence it may be desirable tominimize the extent of swelling in Li/CF_(x) electrochemical cells.

SUMMARY

In one embodiment, the present invention describes an electrochemicalcell that converts chemical energy to electrical energy. Particularly,the invention pertains to an electrochemical cell having a cathode withan active material of fluorinated carbon on a perforated metal cathodecurrent collector, a lithium anode on a perforated metal anode currentcollector, a stepped header, a stable electrolyte, and a separator. Invarious embodiments, the invention provides an anode current collectordesign, a cathode current collector design, a stepped header design, acathode formulation, an electrolyte formulation, a separator, and abattery incorporating the electrochemical cell.

In one embodiment, the swelling of the cell after discharge to zero voltis less than or equal to about 2 percent.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 illustrates a perspective view of a finished electrochemicalcell, in accordance with an embodiment of the present invention;

FIG. 2 illustrates an exploded view of an electrochemical cell, inaccordance with an embodiment of the present invention;

FIG. 3 illustrates a cross-sectional view of an electrochemical cell, inaccordance with an embodiment of the present invention;

FIG. 4 illustrates a cathode current collector of an electrochemicalcell, in accordance with an embodiment of the present invention;

FIG. 5 illustrates an anode current collector of an electrochemicalcell, in accordance with an embodiment of the present invention;

FIG. 6 illustrates an exploded view of an anode including an anodecurrent collector and two lithium foils of an electrochemical cell, inaccordance with an embodiment of the present invention;

FIG. 7 illustrates a stepped header of an electrochemical cell, inaccordance with an embodiment of the present invention;

FIG. 8 is a graph illustrating a discharge curve of an Li/CF_(x)electrochemical cell, constructed in accordance with embodiments of thepresent invention;

FIG. 9 is a graph illustrating deep discharge of an Li/CF_(x) anelectrochemical cell, constructed in accordance with embodiments of thepresent invention; and

FIG. 10 is a graph illustrating a degree of swelling of twenty-fourLi/CF_(x) electrochemical cells, constructed in accordance withembodiments of the present invention.

Unless otherwise indicated illustrations in the figures are notnecessarily drawn to scale.

DETAILED DESCRIPTION

The present invention is best understood by reference to the detailedfigures and description set forth herein.

Embodiments of the invention include a primary lithium-basedelectrochemical cell. It may be appreciated that those skilled in theart will, in light of the teachings of the present invention, understandthat the term “primary” denotes a non-rechargeable electrochemical cell,in contrast to the term “secondary” which denotes a rechargeableelectrochemical cell. As used herein, a battery, may consist of one ormore of the primary electrochemical cells. Typically, primary lithiumbatteries are those having metallic lithium anode, pairing with variouscathodes, including Li/CF_(x), Li/MnO₂, Li/SVO, and Li/Hybrid, whereHybrid is a mixture of CF_(x), and/or MnO₂, and/or SVO.

During the discharge of such a battery, the oxidation of the lithiummetal to lithium ions takes place at the anode according to thefollowing reaction:

Li→Li⁺ +e

The reduction of the oxidizing substance occurs at the cathode. In thecase where the oxidizing agent is CF_(x), the reduction reaction is asfollows:

CF_(x) +e+xLi⁺→C+xLiF

During discharge, the oxidation of the lithium metal to lithium ionsoccurs at the anode, and the lithium ions leave anode surface andmigrate into the porous cathode. At the cathode during discharge, theinsertion of lithium into CF_(x) takes place, producing insolublelithium fluoride and graphite (an electronic conductor).

For example, a theoretical calculation on electrode dimension changeduring discharge may be done in the following manner. Carbonmonofluoride (CF_(x)) is used as the cathode active material for thepresent inventions. The overall discharge reaction in a Li/CF_(x) cellis shown in the following equation I.

xLi+CF_(x)→C+xLiF  (Equation 1)

Table 1 provided below shows the data for volume expansion for thecathode, in an exemplary embodiment. Based on the data in Table 1, forthe discharge reaction given in equation (1) with the molar volume ofCF_(1.0) at 11.2 cubic centimeter per mole (2.8 grams per cubiccentimeter), LiF at 9.8 centimeter per mole (2.65 grams per cubiccentimeter), and C at 6.0 centimeter per mole (2.0 grams per cubiccentimeter), the volume expansion for the cathode may be calculated toabout 41 percent for a complete discharge. On the other hand, the Lianode will be completely dissolved by anodic reaction, as shown in Table1, and the volume expansion of anode is about −100 percent (minus 100percent). Based on the molar volume of each species in equation (1), ifone combines the volume changes on both cathode side and anode side, thenet expansion for the whole cell (as provided in Table 1) is about −34.7percent (minus 34.7 percent), assuming the capacity ratio of anode tocathode is 1:1.

TABLE 1 Theoretical calculation for cell dimension change duringdischarge Density Molar Volume Volume Reaction Species (g/cm³)(cm³/mole) Expansion* Cathode CF_(1.0) 2.8 11.2     41% CF_(1.0) → C 2.06.0 (6.0 + 9.8 − 11.2)/ C + LiF LiF 2.65 9.8 11.2 Anode Li 0.534 13.0 −100% Li → Li⁺ Cell NA NA NA −34.7% CF_(1.0) + Li → (6.0 + 9.8 − 11.2 −C + LiF 13.0)/(11.2 + 13.0) *Assuming the capacity ratio of anode tocathode is 1:1

One skilled in the art may appreciate that the above calculation takesinto account only the active materials in the electrochemical cell. Itdoes not consider change in the volume of cathode binder and cathodeconductive filler and change in the gap between CF_(x) particles andcarbon particles. However, the negative volume change in the solid phasemay create more void space between solid particles. Therefore, whiledischarge proceeds there may be a tendency of lack of electrolytebetween the solids because the electrolyte volume is fixed and is equalto the initial value at undischarged state if the side reaction forelectrolyte during the discharge is negligible. The above descriptionsabout change in the volume of reactants and products imply that theelectrochemical reaction itself may not be the cause of the swelling ofa Li/CF_(x) cell. Instead, it is the electrochemical reaction that maylead to a shrinking of a Li/CF_(x) cell. Accordingly, in variousembodiments, if the factors, such as selection of cathode activematerial, optimized cathode and anode design, optimized value ofelectrolyte amount, are appropriately determined, the cell swelling maybe minimized.

Embodiments of the invention are described below with reference to theFigures, experimental, and detailed description. However, those skilledin the art will readily appreciate that the detailed description givenherein with respect to these figures and experimental is for explanatorypurposes as the invention extends beyond these limited embodiments.

The singular forms “a”, “an” and “the” include plural referents unlessthe context clearly dictates otherwise.

The term “optional” or “optionally” means that the subsequentlydescribed event or circumstance may or may not occur, and that thedescription includes instances where the event occurs and instanceswhere it does not.

In one embodiment is provided, an electrochemical cell. Theelectrochemical cell includes a cathode, an anode, a header, and anelectrolyte. The cathode includes a cathode formulation. The cathodeformulation includes a cathode active material, a conductive carbonfiller, and a binder. The cathode formulation is disposed on a cathodecurrent collector. The anode comprises at least two lithium metal foilsdisposed on an anode current collector. The header includes a steppedheader. The header includes at least two steps, wherein the first stepis to fulfill the ball seal requirements, and the second step is tofulfill the glass sealing requirements. The electrolyte comprises alithium salt in a mixed solvent. The ratio of an amount of electrolyteto an amount of cathode active material is about 0.7 to about 1.1. Thecell has a swelling percentage of less than or equal to about 2 percent.

Referring to FIG. 1, a perspective view 100 of a finishedelectrochemical cell is illustrated, in accordance with an embodiment ofthe present invention. The electrochemical cell includes an outer casing110, and a header 112. The header 112 includes a vent location 118, andpins 114 and 116 for external connection. Internally the pin 114 isconnected to the cathode current collector (not shown in figure) and thepin 116 is connected to the anode current collector (not shown infigure).

In one embodiment, the cathode includes a cathode current collector. Itmay be appreciated that those skilled in the art will, in light of theteachings of the present invention, that the cathode current collectormay include any suitable material known to be used in the art as acathode current collector. Suitable materials may include, but are notlimited to, stainless steel, aluminum, and titanium. In an exemplaryembodiment, the material used for the cathode current collector isstainless steel, such as, for example, SS316, SS316L, SS304.

In one embodiment, the cathode current collector is perforated. In anexemplary embodiment, the perforation consists of large circles andsmall circles in order to maximize the void area while maintaining thecurrent collector strength. The maximized void area is beneficial forenhancing the adhesion between the two halves of the cathode pelletsandwiching the current collector. The ratio of number of large circlesto small circles is about 4:3. Alternatively, the void area can takeother shapes, such as square, diamond, rectangular, and triangle. In oneembodiment, the diameter for the large circles may be in a range ofabout 3.0 millimeter (mm) to about 2.0 mm. In another embodiment, thediameter for the large circles may be in a range of about 2.8 mm toabout 2.2 mm. In yet another embodiment, the average diameter for thelarge circles may be in a range of about 2.6 mm to about 2.3 mm. In oneembodiment, the average diameter for the large circles is about 2.4 mm.In one embodiment, the diameter for the small circles may be in a rangeof about 1.4 mm to about 2.5 mm. In another embodiment, the diameter forthe small circles may be in a range of about 1.6 mm to about 2.3 mm. Inyet another embodiment, the average diameter for the small circles maybe in a range of about 1.8 mm to about 2.1 mm. In one embodiment, theaverage diameter for the small circles is about 1.9 mm.

As shown herein below with reference to FIG. 4, in one embodiment, theratio of perforated area to the whole cathode current collector(excluding the tabbing area) may be in a range of about 0.40 to about0.80 In another embodiment, the ratio of perforated area to the wholecathode current collector (excluding the tabbing area) may be in a rangeof about 0.50 to about 0.70 In yet another embodiment, the ratio ofperforated area to the whole cathode current collector (excluding thetabbing area) may be in a range of about 0.55 to about 0.65 In oneembodiment, the ratio of a perforated area to a whole area of cathodecurrent collector (excluding the tabbing area) is about 0.60.

In one embodiment, the cathode current collector has a thickness. In oneembodiment, the thickness of the cathode current collector may be in arange of about 0.002 mm to about 0.010 mm. In another embodiment, thethickness of the cathode current collector may be in a range of about0.040 mm to about 0.090 mm. In yet another embodiment, the thickness ofthe cathode current collector may be in a range of about 0.060 mm toabout 0.080 mm. In one embodiment, the thickness of the cathode currentcollector is about 0.075 mm.

In one embodiment, the cathode formulation comprises a cathode activematerial, at least one conductive carbon filler, and a binder. In oneembodiment, the cathode active material employed in the cathodeformulation includes electrochemically active fluorinated carbon, i.e.,CF_(x). In one embodiment, the CF_(x) material may be blended with thebinder and the conductive carbon to form a pellet. The pellet may thenbe disposed onto the cathode current collector, i.e., the pellet may bepressed onto the cathode current collector. In one embodiment, theconductive carbon filler may include carbon black.

Accordingly, in one embodiment, the cathode active material comprisesfluorinated carbons represented by the formula CF_(x), wherein x is anumber between 0.1 and 2.0. The atomic weight of fluorine is 18.998 andthe atomic weight of carbon is 12.011. The fluorination level of a givenCF_(x) material may be expressed as a percentage that represents theatomic weight contribution of the fluorine (18.998×) divided by the sumof the atomic weight contribution of the fluorine (18.998×) and theatomic weight contribution of the carbon (12.011). Thus, for C₁F₁stoichiometry, the fluorination level would be18.998/(18.998+12.011)=61.3 percent.

CF_(x) is conventionally prepared from the reaction of fluorine gas witha crystalline or amorphous carbon. Graphite is an example of acrystalline form of carbon, while petroleum coke, coal coke, carbonblack and activated carbon are examples of amorphous carbon. Thereaction between fluorine and carbon is usually carried out attemperatures ranging from 300 degrees Celsius to 650 degrees Celsius ina controlled pressure environment. A variety of CF_(x) materials areavailable from commercial sources, including materials derived from thefluorination of petroleum coke, carbon black and graphite.

Suitable examples of fluorinated carbons that may be used in forming acathode as disclosed herein include, but are not limited to, fluorinatedcarbons that are based on different carbonaceous starting materials. Forexample, a cathode in accordance with the invention can be formed by afluorinated petroleum coke. The fluorinated petroleum coke for use inthe present invention is preferably fully fluorinated to a fluorinationlevel of approximately 58 to 65 percent, with x value between 0.9 to1.2. However, other fluorination levels could potentially also be used.Advantages of using petroleum coke based CF_(x) material is that it isthermally stable in contact with electrolyte in a wide temperature rangeof about −40 degrees Celsius to about 70 degrees Celsius. The petroleumcoke based CF_(x) material is also found to be chemically stable incontact with electrolyte, leading to minimal or no side reactions thatmay generate gas species causing cell swelling. Suitable examples of theCF_(x) material include but are not limited to Carbofluor® 1000 fromAdvanced Research Chemicals (Catoosa Okla.).

In one embodiment, as mentioned hereinabove, cathodes may include theusual non-electrochemically active materials, such as conductive fillersand a binder. In one embodiment, the conductive filler is carbon black,although graphite or mixtures of carbon black and graphite may also beused. In one embodiment, the conductive carbon filler used in thecathode formulation is also thermally and chemically stable. Suitableexamples of the conductive carbon filler include, but are not limitedto, Super P®-Li from TIMCAL. Metals such as nickel, aluminum, titaniumand stainless steel in powder form may likewise be used. Suitableexamples of binder include but is not limited to an aqueous dispersionof a fluorinated resin material, such as a polytetrafluoroethylene(PTFE) or polyvinylidene fluoride (PVDF). In one embodiment, the bindingmaterial is inert PTFE emulsion. It may be appreciated that thoseskilled in the art will, in light of the teachings of the presentinvention, that any suitable mixing ratio of the fluorinated carbon, theconductive filler, and the binder may be used. In an exemplaryembodiment, the cathode may include, by weight, 90 percent of thefluorinated carbon material, 6.0 percent conductive filler and 4.0percent binder.

During fabrication of the CF_(x) cathode, the fluorinated carbonmaterial, which comes in powder form, is blended with the conductivefiller. The CF_(x) and conductive filler are then combined with thebinder by a wet process. The wetted cathode mixture is intimatelyblended, filtered and dried, then pressed into a cathode currentcollector as illustrated in FIG. 4. The current collector will assist informing electrical conducting path between cathode and cell positiveterminal and promote uniform utilization of the cathode material duringdischarge.

In one embodiment, the cathode current collector may be coated withconductive carbon. The coating is done before pressing the pellet. Theconductive carbon coating may help to promote adhesion between thepellet (cathode formulation) and the cathode current collector, and toenhance the continuity of electrical conduction between the cathodecurrent collector and the pellet. In one embodiment, the conductivecarbon material may include, but not be limited to, graphite with athermoplastic binder. In one embodiment, the conductive carbon coatingon the cathode current collector may be obtained by application of acoating material such as commercially available Dag® EB-012 by AchesonColloids Company. on the cathode current collector surface. Advantagesof using the conductive coating includes reduction of cathode swelling.In one embodiment, the conductive carbon coating has a thickness. In oneembodiment, the thickness of the conductive carbon coating may be in arange of about 0.040 millimeter (mm) to about 0.0120 mm. In anotherembodiment, the thickness of the conductive carbon coating may be in arange of about 0.050 millimeter (mm) to about 0.100 mm. In yet anotherembodiment, the thickness of the conductive carbon coating may be in arange of about 0.060 millimeter (mm) to about 0.090 mm. In oneembodiment, the thickness of the conductive carbon coating is about0.080 mm.

In various embodiments, advantages of using a perforated cathode currentcollector include improved pellet cohesion around the edges of theperforations. Further the alignment tab, as described in FIG. 4,features a partially etched cut line which facilitates consistent pelletpressing while minimizing final tab length and interference with the tabto the header weld.

In one embodiment, the anode includes at least one lithium foil disposedon an anode current collector. It may be appreciated that those skilledin the art will, in light of the teachings of the present invention,that the anode current collector may include any suitable material knownto be used in the art as an anode current collector. Suitable materialsmay include, but are not limited to, stainless steel, and copper. In anexemplary embodiment, the material used for the anode current collectoris stainless steel, such as SS316, SS316L and SS304, as it has a highstrength, high stability toward lithium metal and electrolyte, and goodelectric conductivity. In one embodiment, the anode current collectormay include a perforated metal, an expanded metal, a grid, or a metallicfabric.

In one embodiment, the perforation consists of a diamond shape, acircle, an oval, a rectangle, a star, a triangle, and combinationsthereof. In one embodiment, the average size of the perforation may bein a range of about 0.10 mm to about 0.20 mm. In another embodiment, theaverage size of the perforation may be in a range of about 0.12 mm toabout 0.18 mm. In yet another embodiment, the average size of theperforation may be in a range of about 0.13 mm to about 0.17 mm. In oneembodiment, the average size of the perforation is about 0.15 mm.

As shown herein below with reference to FIG. 5, in one embodiment, thepercentage of perforated area to the whole anode current collector(excluding the tabbing area) may be in a range of about 30 percent toabout 90 percent. In another embodiment, the percentage of perforatedarea to the whole anode current collector (excluding the tabbing area)may be in a range of about 40 percent to about 80 percent. In yetanother embodiment, the percentage of perforated area to the whole anodecurrent collector (excluding the tabbing area) may be in a range ofabout 50 percent to about 70 percent. In one embodiment, the percentageof a perforated area to a whole area of anode current collector(excluding the tabbing area) is about 60 percent. The advantage of theanode current collector is that, it may allow uniform utilization oflithium foils during discharge. At the same time, the perforated anodecurrent collector may take up only a little amount of volume inside thecell, allowing maximization of the amount of electrochemically activecomponents in the cell to generate high energy density.

In one embodiment, the total surface area of the anode current collectorexcluding the central folding and tabbing area is equal to or a littlesmaller than the area of the lithium foils. In one embodiment, the ratioof the surface area of the current collector (excluding the centralfolding and tabbing area) to the area of the lithium foils may be in arange of about 70 percent to about 100 percent. In another embodiment,the ratio of the surface area of the current collector (excluding thecentral folding and tabbing area) to the area of the lithium foils maybe in a range of about 80 percent to about 100 percent. In yet anotherembodiment, the ratio of the surface area of the current collector(excluding the central folding and tabbing area) to the area of thelithium foils may be in a range of about 90 percent to about 100percent. In one embodiment, the anode current collector has a thickness.

In one embodiment, the thickness of the anode current collector may bein a range of about 0.010 mm to about 0.100 mm. In another embodiment,the thickness of the anode current collector may be in a range of about0.020 mm to about 0.070 mm. In yet another embodiment, the thickness ofthe anode current collector may be in a range of about 0.040 mm to about0.060 mm. In one embodiment, the thickness of the anode currentcollector is about 0.050 mm.

Referring to FIG. 2, an exploded view 200 of an electrochemical cell isillustrated, in accordance with an embodiment of the present invention.The view 200 shows a cell container or casing 210 and a blow-up portion(Detail A 212) of header 214. The detail A 212 shows the header 214 toinclude an opening 217 for receiving a ball seal 218 which may then besealed with a fill port cover 216, openings 224 and 226 for connecting atab portion 229 of a cathode current collector 228 and a tab portion 239of an anode current collector 238 to pin extenders 220 and 222respectively. In one embodiment, the pin extenders 220 and 22 may begold plated. The opening 217 may function as a vent in the cell. Theview 200 also shows a cathode current collector 228, more particularlythe tab portion 229 of the cathode current collector 228, encased in thecathode formulation in the form of a cathode pellet 230, a cathodeseparator pouch 232 encasing the cathode current collector 228 and thecathode pellet 230, two lithium foils 234, 236, an anode currentcollector 238 with the tab portion 239, an anode separator pouch 240,and an insulator pouch 242 that contains and insulates all the parts ofthe cell from the outer casing 210. The negative current output terminali.e., pin extender 222 of the cell may be connected to the negativeterminal pin, then connected to anode current collector tab portion 239.

Referring to FIG. 4, a cathode current collector of an electrochemicalcell is illustrated, in accordance with an embodiment of the presentinvention. As shown in FIG. 4, the cathode current collector 400includes a perforated stainless-steel plate that includes large circles410 and small circles 412. As mentioned herein above, in one exemplaryembodiment, the average diameter for the large circles is about 2.4 mm,the average diameter of the small circles is about 1.9 mm and the ratioof perforated area 410, 412 to the whole collector 414 (excluding thetabbing area 416) is 0.6.

Referring to FIG. 5, an anode current collector of an electrochemicalcell is illustrated, in accordance with an embodiment of the presentinvention. As shown in FIG. 5, the anode current collector 500 includescentral portion 514 having two side perforated side portions 510, 512,and a tab portion 516 connected to one of the side portions. The anodecurrent collector 500, in an exemplary embodiment is folded along thecentral alignment feature 514 along central axis 518 to form a book likestructure shown in view 520. In one embodiment, the alignment feature514 in the center of the anode current collector, may facilitate properanode to anode current collector alignment and anode current collectorfolding, which are key steps in the cell construction. As described withreference to FIG. 3 hereinbelow, the anode current collector (anode) andthe cathode current collector (cathode) in the cell are assembled in amanner such that the anode current collector sandwiches the cathodecurrent collector. The two holes in the center of the anode currentcollector will allow the anode current collector to sit on a fixturestationary, and lithium foils can be pressed properly onto the anodecurrent collector. Also, the two holes void of materials allow for easyfolding of the anode current collector to form proper geometrysandwiching the cathode to fit into the cell case.

Referring to FIG. 6, an exploded view of an anode including an anodecurrent collector and two lithium foils of an electrochemical cell isillustrated, in accordance with an embodiment of the present invention.As shown in FIG. 6, lithium foils 612, 614 are then disposed onto thefolded anode current collector 610 with sides 510, 512, the centralalignment feature 514. In one embodiment, the lithium foils are pressedon to the perforated surface of the anode current collector. Likewise,other techniques now known by those skilled in the art, or laterdeveloped, may be applied to dispose the lithium foil on to currentcollector.

In one embodiment, the electrochemical cell disclosed herein includes astepped header design. In one embodiment, stepped header design consistsof two or more steps in the header body profile. Referring to FIG. 7 isillustrated a stepped header of an electrochemical cell, in accordancewith an embodiment of the present invention. As shown in view 700 inFIG. 7, the lower portion 710 of the header includes a first step 712,and a second step 714. The optimized stepped header design of the headermay allow for increased or maximum internal cell volume. In oneexemplary embodiment, the first step 712 may be designed around ballseal requirements and the second step 714 may be designed aroundglass-to-metal seal requirements. The first step 712 should havesufficient thickness so that the contact area of ball to header isadequate to hold the ball in place. The ball can be as small as possibleso that the thickness of this step of header can be smaller than thethickness of the second step, thus yielding more cell internal volume.The first step includes an opening 716 which may be designed to receivethe ball seal 218 and the fill port cover 216, described hereinabove,with reference to FIG. 2. The second step 714 step should havesufficient thickness so that the glass in the glass-to-metal seal canhave sufficient thickness to form a hermetic seal. The second step 716includes terminal pins 718 and 720 for connecting the tab portion 229 ofthe cathode current collector 228 and the tab portion 239 of the anodecurrent collector 240 to pin extenders 220 and 222 respectively, asdescribed hereinabove with reference to FIG. 2.

In one embodiment, the thickness of the first step may be in a range ofabout 0.7 mm to about 1.5 mm. In another embodiment, the thickness ofthe first step may be in a range of about 0.8 mm to about 1.4 mm. In yetanother embodiment, the thickness of the first step may be in a range ofabout 0.9 mm to about 1.3 mm. In one embodiment, the first step of theheader may have a thickness of about 1.1 mm. In one embodiment, thethickness of the second step may be in a range of about 1.1 mm to about1.9 mm. In another embodiment, the thickness of the second step may bein a range of about 1.2 mm to about 1.8 mm. In yet another embodiment,the thickness of the second step may be in a range of about 1.3 mm toabout 1.7 mm. In one embodiment, the second step of the header may havea thickness of about 1.5 mm.

Advantages of the stepped header design include an increased internalvolume of the electrochemical cell, the utilization of which allows thecell to achieve electrolyte volume and void volume goals. It may beappreciated that those skilled in the art will, in light of theteachings of the present invention, that a proper selection of theamount of electrolyte and void volume may positively impact the cellenergy density and the cell swelling. A sufficient amount of electrolyteis necessary for the cell to deliver desirable energy. But over fill ofelectrolyte in the cell may increase the risk of cell swelling becausethere will be less void volume that can be used for holding gas speciesformed as a result of side reactions in the cell. In one embodiment, theamount of electrolyte filled in a cell may be in a range of about 38percentage to about 46 percentage based on the total internal volume ofthe cell. In another embodiment, the amount of electrolyte filled in acell may be in a range of about 40 percentage to about 44 percentagebased on the total internal volume of the cell. In yet anotherembodiment, the amount of electrolyte filled in a cell may be in a rangeof about 41 percentage to about 43 percentage based on the totalinternal volume of the cell. In one embodiment, the amount ofelectrolyte filled in a cell is about 42 percentage based on the totalinternal volume of the cell.

In one embodiment, the amount of void volume in a cell may be in a rangeof about 3 percentage to about 9 percentage based on the total internalvolume of the cell. In another embodiment, the amount of void volume ina cell may be in a range of about 4 percentage to about 8 percentagebased on the total internal volume of the cell. In yet anotherembodiment, the amount of void volume in a cell may be in a range ofabout 5 percentage to about 7 percentage based on the total internalvolume of the cell. In one embodiment, the amount of void volume in acell is about 6 based on the total internal volume of the cell.

In one embodiment, the ratio of the electrolyte volume to the voidvolume in a newly manufactured cell may be in a range of about 4.0 toabout 10.0 based on the total volume of the cell. In another embodiment,the ratio of the electrolyte volume to the void volume in a newlymanufactured cell may be in a range of about 5.0 to about 9.0 based onthe total volume of the cell. In yet another embodiment, the ratio ofthe electrolyte volume to the void volume in a newly manufactured cellmay be in a range of about 6.0 to about 8.0 based on the total volume ofthe cell. In one embodiment, the ratio of the electrolyte volume to thevoid volume in a newly manufactured cell may be about 7.0 based on thetotal volume of the cell.

In one embodiment, the electrolyte formulation includes a lithium saltin a mixed solvent. The electrolyte provides an ionic source serving asan electrical conducting carrier between the cathode and the anodeduring cell discharge. Suitable lithium salts may include, but are notlimited to, LiBF₄, LiAsF₆, LiSbF₆, or LiClO₄, or a combination of two ormore of these salts. In one embodiment, the solvent may include amixture of two compounds. One compound having a low viscosity and theother compound having a high permittivity. Suitable examples of solventshaving a low viscosity include, but are not limited to,1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE),1-ethoxy,2-methoxyethane (EME), ethyl methyl carbonate, methyl propylcarbonate, ethyl propyl carbonate, and diethyl carbonate. Suitableexamples of solvents having a high permittivity include, but are notlimited to, propylene carbonate (PC), ethylene carbonate (EC),γ-butyrolactone (GBL), and N-methyl-pyrrolidinone (NMP). In oneembodiment, the lithium salt is LiBF₄, and the mixed solvent is composedof 1,2-dimethoxyethane (DME) and γ-butyrolactone (GBL). It may beappreciated that those skilled in the art will, in light of theteachings of the present invention, that the selected electrolyte mayhave a good electrical conductivity and chemical stability when incontact with both lithium anode and CF_(x) cathode, thus aiding inelimination or minimizing of cell swelling.

In one embodiment, the electrolyte amount may be determined by the ratioof electrolyte amount to the amount of fluorinated carbon (CF_(x)). Tomaximize the energy density, as-much-as-possible electrochemical activematerials should be filled into a cell. At the same time, each bit ofthe cathode active material should be in contact with the electrolyte,in order for each part of cathode to be active in contributing todischarge capacity. It may be appreciated that those skilled in the artwill, in light of the teachings of the present invention, that adequateamount of electrolyte is needed to achieve high energy density. However,more than sufficient amount of electrolyte may cause lack of void volumein the cell, thus leading to cell swelling. It may be appreciated thatthose skilled in the art will, in light of the teachings of the presentinvention, that the ratio of electrolyte amount to the amount offluorinated carbon (CF_(x)) should be properly determined. In oneembodiment, the ratio of electrolyte amount to the amount of fluorinatedcarbon (CF_(x)) is in a range of about 0.7 to about 1.1. In anotherembodiment, the ratio of electrolyte amount to the amount of fluorinatedcarbon (CF_(x)) is in a range of about 0.8 to about 1.0. In yet anotherembodiment, the ratio of electrolyte amount to the amount of fluorinatedcarbon (CF_(x)) is in a range of about 0.9 to about 1.05. In oneembodiment, the ratio of electrolyte amount to the amount of fluorinatedcarbon (CF_(x)) is about 0.93.

In one embodiment, the ratio of solvent one with low viscosity and thesolvent two with high permittivity is, by volume, in a range of about0.5 to about 1.5 In another embodiment, the ratio of solvent one withlow viscosity and the solvent two with high permittivity is in a rangeof about 0.7 to about 1.3 In yet another embodiment, the ratio ofsolvent one with low viscosity and the solvent two with highpermittivity is in a range of about 0.8 to about 1.2 In one embodiment,the ratio of solvent one with low viscosity and the solvent two withhigh permittivity is about 1.0.

In one embodiment, the amount of lithium salt to solvent mixture is in arange of about 0.8 moles per liter to about 1.2 moles per liter based ona total volume of the solvent. In another embodiment, the ratio oflithium salt to solvent mixture is in a range of about 0.9 moles perliter to about 1.1 moles per liter based on a total volume of thesolvent. In yet another embodiment, the ratio of lithium salt to solventmixture is in a range of about 0.95 moles per liter to about 1.05 molesper liter based on a total volume of the solvent. In one embodiment, theratio of lithium salt to solvent mixture is about 1.0 moles per liter.

Referring to FIG. 3, a cross-sectional view of an electrochemical cellis illustrated, in accordance with an embodiment of the presentinvention. In the exemplary embodiment illustrated in FIG. 3, the view300 of the electrochemical cell includes a cell case or battery case 328which houses a cathode assembly including a cathode current collector318 integrated with a cathode current collector tab 310 and encased in acathode separator 312. The cathode pellet 322 is pressed on to thecathode current collector 318. The cell also includes an anode assemblyincluding an anode 320 pressed on to the anode current collector 324 andencased in an anode separator 316. The anode assembly sandwiches thecathode assembly. The anode assembly and the cathode assembly are thenencased in an insulator pouch 326 which is covered by the cell casing328. The cell may be closed with a stepped header 330 which is welded onits circumference with the cell casing by forming welding rings 332. Theheader includes a feed thru pin 334 for connection to device and glassto metal seal 336 to prevent leakage of electrolyte, solvents, etc. Inthe exemplary embodiment illustrated in FIG. 3, the cathode currentcollector has a thickness of about 0.075 mm, is made of stainless steel,has perforations formed of large circles (average diameter of about 2.4mm) and small circles (average diameter of about 1.9 mm), cathode pellet322 is composed of fluorinated carbon (CF_(x)), carbon black and binderin a ratio of about 90:6:4, ratio of perforated area to total area ofcathode current collector is about 0.6 and cathode is coated with a 0.05mm thick layer of conductive carbon (not shown in figure). The anodecurrent collector includes a lithium metal foil 320 pressed on the anodecurrent collector 324, the anode current collector has a thickness of0.05 mm, is made of stainless steel, has diamond shaped perforations,and ratio of perforated area to total area of anode current collector isabout 0.6. The electrolyte used includes lithium salt is LiBF₄, andmixed solvent composed of 1,2-dimethoxyethane (DME) and γ-butyrolactone(GBL), and the ratio of electrolyte amount to the amount of fluorinatedcarbon (CF_(x)) is about 0.93.

In one embodiment, the separator may be selected from those commerciallyavailable separators. As known to those skilled in the art, theseparator is typically an electrically non-conducting porouselectrolyte-filled membrane, which is sandwiched between and in contactwith the cathode and anode. Its role is to prevent direct electroniccontact between cathode and anode, thus avoiding a short-circuit betweenthe two electrodes, to allow the flow of ionic species within the cell.The separator should be chemically stable while in contact with each ofthe cathode, anode and electrolyte. The function and reliability of theseparator is critical for the optimal performance of lithium batteries.The separator affects the internal cell resistance, discharge rates andcell stability. The separator material in this invention is selectedbased on its stability, porosity, thickness and strength, to allow goodionic conductivity as well as to maintain stability. In one embodiment,the separator may have a thickness in a range of about 0.010 mm to about0.035 mm. In one embodiment, the separator may have a porosity of about40 percent to about 60 percent. In various embodiments, separator mayinclude one layer of polymer material, or multi-layer polymer materials.Suitable examples of separator material may include, but are not limitedto, monolayer polypropylene, or can be tri-layer that consist of twolayers of polypropylene, and sandwiching monolayer of polyethylene. Thestability of the separator contributes to non-swelling of the lithiumbattery during deep discharge.

According to an embodiment of this invention, the case material (outercasing) for the cell may be made of titanium or stainless steel. In oneembodiment, the case material is titanium, as titanium allows goodcompatibility with body fluid while the battery is implanted into humanbody.

Experimental

Example 1 provides construction details of an anode sample of anelectrochemical cell in accordance with embodiments of the presentinvention.

In Example 1, anode of the electrochemical cell is constructed using twometallic lithium foils and a perforated current collector made ofstainless steel. The stainless steel perforated current collector isperforated. FIG. 5 represents a stainless steel perforated currentcollector as described in Example 1. As shown in FIG. 5, in theexemplary embodiment provided in Example 1, the perforations may consistof diamond shapes. The stainless steel perforated current collectoraccordingly has a void area and a total area. The stainless steelperforated current collect of Example 1, has a ratio of perforated voidarea to the total area of current collector (excluding the centralfolding and tabbing area) of about 0.6. The thickness of the stainlesssteel perforated current collector is about 0.050 millimeters. Thenegative terminal 222 of the electrochemical cell is connected to thetab portion 516, 239 of the stainless steel perforated anode currentcollector.

Example 2 provides construction details and swelling characteristics ofan Li/CF_(x) electrochemical cell in accordance with embodiments of thepresent invention.

One Li/CF_(x) cell was constructed according to preferred embodiments ofthe present inventions as described with reference to FIG. 3hereinabove. The cell was discharged by a 5-day accelerated protocol (asin FIG. 8) to 2.0 Volts. For testing of a medical battery, the testduration of three months to six months is not unusual. The 5-dayprotocol allow a faster output of the testing. The swelling observed forthe cell is about 1.0 percent. As mentioned herein, “swelling” isdefined/calculated as the difference in the cell thickness between thecell in a discharged state and the cell in an undischarged divided bythe thickness of the cell in the undischarged state. Referring to FIG. 8is shown a graph illustrating a discharge of an Li/CF_(x) anelectrochemical cell, constructed in accordance with embodiments of thepresent invention. The graph 800 includes Voltage on Y-Axis 810 andDischarge capacity in percentage on X-axis 812. The Cell constructed inExample 2, was discharged from 2.5 V to 2.0 V in a 5-day acceleratedprotocol and the percentage of discharge capacity was plotted as curve814.

Example 3 provides construction details and swelling characteristics oftwo Li/CF_(x) electrochemical cells in accordance with embodiments ofthe present invention.

Two Li/CF_(x) cells were constructed according to preferred embodimentsof the present inventions as described with reference to Example 2above. The two cells were discharged under a 5-day accelerated protocol.Referring to FIG. 9 is shown a graph illustrating deep discharge of twoLi/CF_(x) electrochemical cells, constructed in accordance withembodiments of the present invention. The graph 900 includes Voltage onY-Axis 910 and Discharge capacity on X-axis 912. The Cells constructedin Example 3, were discharged from 2.5 V to 2.0 V in a 5-day acceleratedprotocol and the discharge capacity was plotted as curve 914 for cell 1,and curve 916 for cell 2. After discharge of the two cells to 0.01V (asshown in FIG. 9), the cell swelling was calculated as about 0.5 percentfor cell 1, and about 1.0 percent for cell 2, when calculated asdescribed hereinabove in Example 2 in comparison to the dimensions ofundischarged cells.

Example 4 provides construction details and swelling characteristics oftwenty-four Li/CF_(x) electrochemical cells in accordance withembodiments of the present invention.

Twenty-four Li/CF_(x) cells were constructed according to preferredembodiments of the present inventions as described with reference toExample 2 above. These twenty-four cells were first discharged to 2.0Volts by an accelerated protocol and the cell thickness was measured atthis stage. The cells were then discharged to 0.0 Volts at 250 microAmperes, and the cell thickness was measured again. Referring to FIG. 10is a shown graph illustrating a degree of swelling of twenty-fourLi/CF_(x) electrochemical cells, constructed in accordance withembodiments of the present invention. The graph 1000 includes cellthickness change in percentage on Y-Axis 1010 and cell group by cellmilliamp hour X-axis 1012. The Cells constructed in Example 4, weredischarged as described herein. FIG. 10 summarizes the swelling data ofthe twenty-four cells while the cells were discharged to 2.0 V andfurther to 0.0 V. The swelling at 2.0 Volts for the 100 percent milliamphour group is only about 1.5 percent. As observed in FIG. 10, there is ageneral trend that the swelling of cell after discharge to 0.0 Volts islesser than that after discharge to 2.0 Volts, even some cells shrunkafter discharge to 0.0V (see 90 percent milliamp hour group in FIG. 10).This may be attributed to the fact that the density of the dischargeproduct i.e., carbon and LiF is greater than the density of thereactants i.e., Li and CF_(x), and thus less volume is needed to holdthe solids inside the container. Further, the internal pressure of thecell is less than the external air pressure, causing the shrinking ofthe cell, and hence a reduction in the cell thickness.

In one embodiment, the electrochemical cell disclosed herein includes anelectrochemical cell with high specific energy, low self-discharge rate,and minimal swelling during deep discharge, particularly for animplantable medical device. For example, the electrochemical cell may beuseful in implantable cardiac monitor (ICM) devices or other implantablemedical products. In various embodiments, the optimized selection ofmaterials, i.e., the materials for cathode, electrolyte, separator,current collector, header, and cell case, and the optimized designs,i.e., the design of the cathode current collector, design of the anodecurrent collector, anode to cathode ratio, electrolyte to cathode ratio,void volume ratio, etc., in the present disclosure may result in reducedgassing and minimal swelling during deep discharge of theelectrochemical cell.

All the features disclosed in this specification, including anyaccompanying abstract and drawings, may be replaced by alternativefeatures serving the same, equivalent or similar purpose, unlessexpressly stated otherwise. Thus, unless expressly stated otherwise,each feature disclosed is one example only of a generic series ofequivalent or similar features.

The foregoing embodiments meet the overall objectives of this disclosureas summarized above. However, it will be clearly understood by thoseskilled in the art that the foregoing description has been made in termsonly of the most preferred specific embodiments. Therefore, many otherchanges and modifications clearly and easily can be made that are alsouseful improvements and definitely outside the existing art withoutdeparting from the scope of the present disclosure, indeed which remainwithin its very broad overall scope, and which disclosure is to bedefined over the existing art by the appended claims.

1. An electrochemical cell comprising: a cathode; an anode; and astepped header comprising a first step and a second step, and a firstopening, a second opening, and a third opening, wherein an upper surfaceof the first step is disposed directly on a lower surface of the headerand an upper surface of the second step is disposed directly on a lowersurface of the first step, the first opening has a length extending froman upper surface of the header, through a thickness of the header and athickness of the first step, and to a bottom surface of the first step,and the second opening and the third opening each have a lengthextending from an upper surface of the header, through the thickness ofthe header, the thickness of the first step, and a thickness of thesecond step, and to a bottom surface of the second step, wherein a firstterminal pin for connection to a cathode tab portion extends through thelength of the second opening and a second terminal pin for connection toan anode tab portion extends through the length of the third opening. 2.The electrochemical cell of claim 1, wherein the first opening in thefirst step is configured for a ball seal.
 3. The electrochemical cell ofclaim 1, wherein the second opening and the third opening in the secondstep are configured for a glass seal.
 4. The electrochemical cell ofclaim 1, wherein the length of the first step is less than a length ofthe header, and a length of the second step is less than a length of thefirst step.
 5. The electrochemical cell of claim 1, wherein the entireupper surface of the first step is disposed directly on the lowersurface of the header and the entire upper surface of the second step isdisposed directly on a lower surface of the first step.
 6. Theelectrochemical cell of claim 1, wherein the electrochemical cell has aninternal volume including an electrode, a separator, an electrolytevolume, and a void volume, and the electrolyte volume is in the range of38 percent to 46 percent of the internal volume.
 7. The electrochemicalcell of claim 6, wherein the electrolyte volume is in the range of about40 percent to about 44 percent of the internal volume.
 8. Theelectrochemical cell of claim 6, wherein the electrolyte volume is inthe range of about 41 percent to about 43 percent of the internalvolume.
 9. The electrochemical cell of claim 6, wherein the electrolytevolume is about 42 percent of the internal volume.
 10. Theelectrochemical cell of claim 6, wherein the void volume is in the rangeof about 3 percent to about 9 percent of the internal volume.
 11. Theelectrochemical cell of claim 6, wherein the void volume is in the rangeof about 4 percent to about 8 percent of the internal volume.
 12. Theelectrochemical cell of claim 6, wherein the void volume is in the rangeof about 5 percent to about 7 percent of the internal volume.
 13. Theelectrochemical cell of claim 6, wherein the void volume is about 6percent of the internal volume.